The Penicillium digitatum protein O-mannosyltransferase Pmt2 is required for cell wall integrity, conidiogenesis, virulence and sensitivity to the antifungal peptide PAF26

Dynamics of interaction and internalisation of the antifungal protein PeAfpA into Penicillium digitatum morphotypes

Antifungal proteins (AFPs) as the highly active PeAfpA from Penicillium expansum or PdAfpB from Penicillium digitatum exert promising antifungal activity, but their mode of action is not fully understood. We characterised the interaction of PeAfpA against P. digitatum, comparing it to the less active PdAfpB. Despite similar effect on conidia germination, PeAfpA did not induce a burst of reactive oxygen species as PdAfpB. Live-cell fluorescence microscopy revealed complex dynamics of interaction and internalisation of both proteins with distinct P. digitatum morphotypes (quiescent conidia, swollen conidia, germlings and hyphae). Labelled PeAfpA co-localised at the cell wall of quiescent conidia, where its localisation was punctate and not uniformly distributed. This pattern changed during germination to a uniform distribution with increased intensity. Conidia from mutants of genes involved in melanin biosynthesis (pksP/alb1 or arp2) showed an altered distribution of PeAfpA but later mimicked the wild type trend of changes during germination. In swollen conidia and germlings, PeAfpA remained attached to the cell wall. In hyphae, PeAfpA was internalised through the growing hyphal tip after binding to the cell wall, in a non-endocytic but energy-dependent process that caused vacuolisation, which preceded cell death. These results may help the development of biofungicides based on AFPs.

Increasing the efficiency of CRISPR/Cas9-mediated genome editing in the citrus postharvest pathogen Penicillium digitatum

BACKGROUND

Penicillium digitatum is a fungal plant pathogen that causes the green mold disease in harvested citrus fruits. Due to its economical relevance, many efforts have focused on the development of genetic engineering tools for this fungus. Adaptation of the CRISPR/Cas9 technology was previously accomplished with self-replicative AMA1-based plasmids for marker-free gene editing, but the resulting efficiency (10%) limited its practical implementation. In this study, we aimed to enhance the efficiency of the CRISPR/Cas9-mediated gene editing in P. digitatum to facilitate its practical use.

RESULTS

Increasing the culture time by performing additional culture streaks under selection conditions in a medium that promotes slower growth rates significantly improved the gene editing efficiency in P. digitatum up to 54-83%. To prove this, we disrupted five candidate genes that were chosen based on our previous high-throughput gene expression studies aimed at elucidating the transcriptomic response of P. digitatum to the antifungal protein PdAfpB. Two of these genes lead to visual phenotypic changes (PDIG_53730/pksP, and PDIG_54100/arp2) and allowed to start the protocol optimization. The other three candidates (PDIG_56860, PDIG_33760/rodA and PDIG_68680/dfg5) had no visually associated phenotype and were targeted to confirm the high efficiency of the protocol.

CONCLUSION

Genome editing efficiency of P. digitatum was significantly increased from 10% to up to 83% through the modification of the selection methodology, which demonstrates the feasibility of the CRISPR/Cas9 system for gene disruption in this phytopathogenic fungus. Moreover, the approach described in this study might help increase CRISPR/Cas9 gene editing efficiencies in other economically relevant fungal species for which editing efficiency via CRISPR/Cas9 is still low.

Studies on the biological role of the antifungal protein PeAfpA from Penicillium expansum by functional gene characterization and transcriptomic profiling

Antifungal proteins (AFPs) from filamentous fungi have enormous potential as novel biomolecules for the control of fungal diseases. However, little is known about the biological roles of AFPs beyond their antifungal action. Penicillium expansum encodes three phylogenetically different AFPs (PeAfpA, PeAfpB and PeAfpC) with diverse profiles of antifungal activity. PeAfpA stands out as a highly active AFP that is naturally produced at high yields. Here, we provide new data about the function of PeAfpA in P. expansum through phenotypical characterization and transcriptomic studies of null mutants of the corresponding afpA gene. Mutation of afpA did not affect axenic growth, conidiation, virulence, stress responses or sensitivity towards P. expansum AFPs. However, RNA sequencing evidenced a massive transcriptomic change linked to the onset of PeAfpA production. We identified two large gene expression clusters putatively involved in PeAfpA function, which correspond to genes induced or repressed with the production of PeAfpA. Functional enrichment analysis unveiled significant changes in genes related to fungal cell wall remodeling, mobilization of carbohydrates and plasma membrane transporters. This study also shows a putative co-regulation between the three afp genes. Overall, our transcriptomic analyses provide valuable insights for further understanding the biological functions of AFPs.

Novel findings about the mode of action of the antifungal protein PeAfpA against Saccharomyces cerevisiae

Antifungal proteins (AFPs) from filamentous fungi offer the potential to control fungal infections that threaten human health and food safety. AFPs exhibit broad antifungal spectra against harmful fungi, but limited knowledge of their killing mechanism hinders their potential applicability. PeAfpA from Penicillium expansum shows strong antifungal potency against plant and human fungal pathogens and stands above other AFPs for being active against the yeast Saccharomyces cerevisiae. We took advantage of this and used a model laboratory strain of S. cerevisiae to gain insight into the mode of action of PeAfpA by combining (i) transcriptional profiling, (ii) PeAfpA sensitivity analyses of deletion mutants available in the S. cerevisiae genomic deletion collection and (iii) cell biology studies using confocal microscopy. Results highlighted and confirmed the role of the yeast cell wall (CW) in the interaction with PeAfpA, which can be internalized through both energy-dependent and independent mechanisms. The combined results also suggest an active role of the CW integrity (CWI) pathway and the cAMP-PKA signalling in the PeAfpA killing mechanism. Besides, our studies revealed the involvement of phosphatidylinositol metabolism and the participation of ROX3, which codes for the subunit 19 of the RNA polymerase II mediator complex, in the yeast defence strategy. In conclusion, our study provides clues about both the killing mechanism of PeAfpA and the fungus defence strategies against the protein, suggesting also targets for the development of new antifungals. KEY POINTS: • PeAfpA is a cell-penetrating protein with inhibitory activity against S. cerevisiae. • The CW integrity (CWI) pathway is a key player in the PeAfpA killing mechanism. • Phosphatidylinositol metabolism and ROX3 are involved in the yeast defence strategy.

FungalBraid 2.0: expanding the synthetic biology toolbox for the biotechnological exploitation of filamentous fungi

Fungal synthetic biology is a rapidly expanding field that aims to optimize the biotechnological exploitation of fungi through the generation of standard, ready-to-use genetic elements, and universal syntax and rules for contributory use by the fungal research community. Recently, an increasing number of synthetic biology toolkits have been developed and applied to filamentous fungi, which highlights the relevance of these organisms in the biotechnology field. The FungalBraid (FB) modular cloning platform enables interchangeability of DNA parts with the GoldenBraid (GB) platform, which is designed for plants, and other systems that are compatible with the standard Golden Gate cloning and syntax, and uses binary pCAMBIA-derived vectors to allow Agrobacterium tumefaciens-mediated transformation of a wide range of fungal species. In this study, we have expanded the original FB catalog by adding 27 new DNA parts that were functionally validated in vivo. Among these are the resistance selection markers for the antibiotics phleomycin and terbinafine, as well as the uridine-auxotrophic marker pyr4. We also used a normalized luciferase reporter system to validate several promoters, such as PpkiA, P7760, Pef1α, and PafpB constitutive promoters, and PglaA, PamyB, and PxlnA inducible promoters. Additionally, the recently developed dCas9-regulated GB_SynP synthetic promoter collection for orthogonal CRISPR activation (CRISPRa) in plants has been adapted in fungi through the FB system. In general, the expansion of the FB catalog is of great interest to the scientific community since it increases the number of possible modular and interchangeable DNA assemblies, exponentially increasing the possibilities of studying, developing, and exploiting filamentous fungi.

Transcriptomic Profile of Penicillium digitatum Reveals Novel Aspects of the Mode of Action of the Antifungal Protein AfpB

Antifungal proteins (AFPs) from filamentous fungi are promising biomolecules to control fungal pathogens. Understanding their biological role and mode of action is essential for their future application. AfpB from the citrus fruit pathogen Penicillium digitatum is highly active against fungal phytopathogens, including its native fungus. Our previous data showed that AfpB acts through a multitargeted three-stage process: interaction with the outer mannosylated cell wall, energy-dependent cell internalization, and intracellular actions that result in cell death. Here, we extend these findings by characterizing the functional role of AfpB and its interaction with P. digitatum through transcriptomic studies. For this, we compared the transcriptomic response of AfpB-treated P. digitatum wild type, a ΔafpB mutant, and an AfpB-overproducing strain. Transcriptomic data suggest a multifaceted role for AfpB. Data from the ΔafpB mutant suggested that the afpB gene contributes to the overall homeostasis of the cell. Additionally, these data showed that AfpB represses toxin-encoding genes, and they suggest a link to apoptotic processes. Gene expression and knockout mutants confirmed that genes coding for acetolactate synthase (ALS) and acetolactate decarboxylase (ALD), which belong to the acetoin biosynthetic pathway, contribute to the inhibitory activity of AfpB. Moreover, a gene encoding a previously uncharacterized extracellular tandem repeat peptide (TRP) protein showed high induction in the presence of AfpB, whereas its TRP monomer enhanced AfpB activity. Overall, our study offers a rich source of information to further advance in the characterization of the multifaceted mode of action of AFPs. IMPORTANCE Fungal infections threaten human health worldwide and have a negative impact on food security, damaging crop production and causing animal diseases. At present, only a few classes of fungicides are available due to the complexity of targeting fungi without affecting plant, animal, or human hosts. Moreover, the intensive use of fungicides in agriculture has led to the development of resistance. Therefore, there is an urgent need to develop antifungal biomolecules with new modes of action to fight human-, animal-, and plant-pathogenic fungi. Fungal antifungal proteins (AFPs) offer great potential as new biofungicides to control deleterious fungi. However, current knowledge about their killing mechanism is still limited, which hampers their potential applicability. AfpB from P. digitatum is a promising molecule with potent and specific fungicidal activity. This study further characterizes its mode of action, opening avenues for the development of new antifungals.

GAR-transferase contributes to purine synthesis and mitochondrion function to maintain fungal development and full virulence of Penicillium digitatum

Penicillium digitatum is one of the most critical phytopathogens during the citrus postharvest period. However, the molecular mechanism of pathogenesis remains to be further explored. Purine is a multiple functional substance in organisms. To verify the role of the de novo purine biosynthesis (DNPB) pathway in P. digitatum, we investigated the third gene Pdgart, glycinamide ribonucleotide (GAR)-transferase, of this pathway in this study. The deletion mutant ΔPdgart was generated in the principle of homologous recombination via Agrobacterium tumefaciens-mediated transformation (ATMT). The phenotypic assay indicated that the ΔPdgart mutant displayed severe defects in hyphae growth, conidiation and germination, which can be rescued by the addition of exogenous ATP and AMP. Compared with wild-type strain N1, the ATP level of strain ΔPdgart was detected to be sharply declined during conidial germination, and this was resulted from the damage to purine synthesis and aerobic respiration. The pathogenicity assay suggested that mutant ΔPdgart infected citrus fruit but attenuated disease, which was owing to its reduced production of organic acids and activities of cell wall degradation enzymes. Additionally, the ΔPdgart mutant showed altered sensitivity to stress agents and fungicides. Taken together, the present study provides insights into the essential functions of Pdgart, and paves the way for further study and novel fungicide development.

Rationally designed antifungal protein chimeras reveal new insights into structure-activity relationship

Antifungal proteins (AFPs) are promising antimicrobial compounds that represent a feasible alternative to fungicides. Penicillium expansum encodes three phylogenetically distinct AFPs (PeAfpA, PeAfpB and PeAfpC) which show different antifungal profiles and fruit protection effects. To gain knowledge about the structural determinants governing their activity, we solved the crystal structure of PeAfpB and rationally designed five PeAfpA::PeAfpB chimeras (chPeAFPV1-V5). Chimeras showed significant differences in their antifungal activity. chPeAFPV1 and chPeAFPV2 improved the parental PeAfpB potency, and it was very similar to that of PeAfpA. chPeAFPV4 and chPeAFPV5 showed an intermediate profile of activity compared to the parental proteins while chPeAFPV3 was inactive towards most of the fungi tested. Structural analysis of the chimeras evidenced an identical scaffold to PeAfpB, suggesting that the differences in activity are due to the contributions of specific residues and not to induced conformational changes or structural rearrangements. Results suggest that mannoproteins determine protein interaction with the cell wall and its antifungal activity while there is not a direct correlation between binding to membrane phospholipids and activity. This work provides new insights about the relevance of sequence motifs and the feasibility of modifying protein specificity, opening the door to the rational design of chimeras with biotechnological applicability.

The Effects of Storage Temperature, Light Illumination, and Low-Temperature Plasma on Fruit Rot and Change in Quality of Postharvest Gannan Navel Oranges

Gannan navel orange (Citrus sinensis Osbeck cv. Newhall) is an economically important fruit, but postharvest loss occurs easily during storage. In this study, the effects of different temperatures, light illuminations, and low-temperature plasma treatments on the water loss and quality of the Gannan navel orange were investigated. The fruit began to rot after 90 d of storage at 5 °C and 20-45 d at 26 °C. Navel oranges stored at 26 °C had 7.2-fold and 3.1-fold higher rates of water loss at the early and late storage stages, respectively, as compared with those stored at 5 °C. Storage at 5 °C decreased the contents of total soluble solids at the early storage stage and the contents of titratable acids at the late storage stage, whereas storage at 26 °C decreased the contents of total soluble solids at the late storage stage and the contents of titratable acids at the early storage stage, respectively. Application of low-temperature plasma produced by air ionization for 6 min, or continuous blue or red light illumination significantly inhibited water loss within 7 and 21 d of storage at 22 °C, respectively, but exhibited no significant effect on fruit quality. Furthermore, the low-temperature plasma treatment protected against fruit rot. Thus, treatment with low-temperature plasma followed by storage at a low temperature under continuous red or blue light illumination was of potential value as a green technology for preserving Gannan navel orange during storage.

Comparative Secretome Analysis of Magnaporthe oryzae Identified Proteins Involved in Virulence and Cell Wall Integrity

Plant fungal pathogens secrete numerous proteins into the apoplast at the plant-fungus contact sites to facilitate colonization. However, only a few secretory proteins were functionally characterized in Magnaporthe oryzae, the fungal pathogen causing rice blast disease worldwide. Asparagine-linked glycosylation 3 (Alg3) is an α-1,3-mannosyltransferase functioning in the N-glycan synthesis of N-glycosylated secretory proteins. Fungal pathogenicity and cell wall integrity are impaired in Δalg3 mutants, but the secreted proteins affected in Δalg3 mutants are largely unknown. In this study, we compared the secretomes of the wild-type strain and the Δalg3 mutant and identified 51 proteins that require Alg3 for proper secretion. These proteins were predicted to be involved in metabolic processes, interspecies interactions, cell wall organization, and response to chemicals. Nine proteins were selected for further validation. We found that these proteins were localized at the apoplastic region surrounding the fungal infection hyphae. Moreover, the N-glycosylation of these proteins was significantly changed in the Δalg3 mutant, leading to the decreased protein secretion and abnormal protein localization. Furthermore, we tested the biological functions of two genes, INV1 (encoding invertase 1, a secreted invertase) and AMCase (encoding acid mammalian chinitase, a secreted chitinase). The fungal virulence was significantly reduced, and the cell wall integrity was altered in the Δinv1 and Δamcase mutant strains. Moreover, the N-glycosylation was essential for the function and secretion of AMCase. Taken together, our study provides new insight into the role of N-glycosylated secretory proteins in fungal virulence and cell wall integrity.

Application of recyclable CRISPR/Cas9 tools for targeted genome editing in the postharvest pathogenic fungi Penicillium digitatum and Penicillium expansum

Penicillium digitatum and Penicillium expansum are plant pathogenic fungi that cause the green and blue mold diseases, respectively, leading to serious postharvest economic losses worldwide. Moreover, P. expansum can produce mycotoxins, which are hazardous compounds to human and animal health. The development of tools that allow multiple and precise genetic manipulation of these species is crucial for the functional characterization of their genes. In this sense, CRISPR/Cas9 represents an excellent opportunity for genome editing due to its efficiency, accuracy and versatility. In this study, we developed protoplast generation and transformation protocols and applied them to implement the CRISPR/Cas9 technology in both species for the first time. For this, we used a self-replicative, recyclable AMA1-based plasmid which allows unlimited number of genomic modifications without the limitation of integrative selection markers. As test case, we successfully targeted the wetA gene, which encodes a regulator of conidiophore development. Finally, CRISPR/Cas9-derived ΔwetA strains were analyzed. Mutants showed reduced axenic growth, differential pathogenicity and altered conidiogenesis and germination. Additionally, P. digitatum and P. expansum ΔwetA mutants showed distinct sensitivity to fungal antifungal proteins (AFPs), which are small, cationic, cysteine-rich proteins that have become interesting antifungals to be applied in agriculture, medicine and in the food industry. With this work, we demonstrate the feasibility of the CRISPR/Cas9 system, expanding the repertoire of genetic engineering tools available for these two important postharvest pathogens and open up the possibility to adapt them to other economically relevant phytopathogenic fungi, for which toolkits for genetic modifications are often limited.

Development of a FungalBraid Penicillium expansum-based expression system for the production of antifungal proteins in fungal biofactories

Fungal antifungal proteins (AFPs) have attracted attention as novel biofungicides. Their exploitation requires safe and cost-effective producing biofactories. Previously, Penicillium chrysogenum and Penicillium digitatum produced recombinant AFPs with the use of a P. chrysogenum-based expression system that consisted of the paf gene promoter, signal peptide (SP)-pro sequence and terminator. Here, the regulatory elements of the afpA gene encoding the highly produced PeAfpA from Penicillium expansum were developed as an expression system for AFP production through the FungalBraid platform. The afpA cassette was tested to produce PeAfpA and P. digitatum PdAfpB in P. chrysogenum and P. digitatum, and its efficiency was compared to that of the paf cassette. Recombinant PeAfpA production was only achieved using the afpA cassette, being P. chrysogenum a more efficient biofactory than P. digitatum. Conversely, P. chrysogenum only produced PdAfpB under the control of the paf cassette. In P. digitatum, both expression systems allowed PdAfpB production, with the paf cassette resulting in higher protein yields. Interestingly, these results did not correlate with the performance of both promoters in a luciferase reporter system. In conclusion, AFP production is a complex outcome that depends on the regulatory sequences driving afp expression, the fungal biofactory and the AFP sequence.

Emerging Roles of Posttranslational Modifications in Plant-Pathogenic Fungi and Bacteria

Posttranslational modifications (PTMs) play crucial roles in regulating protein function and thereby control many cellular processes and biological phenotypes in both eukaryotes and prokaryotes. Several recent studies illustrate how plant fungal and bacterial pathogens use these PTMs to facilitate development, stress response, and host infection. In this review, we discuss PTMs that have key roles in the biological and infection processes of plant-pathogenic fungi and bacteria. The emerging roles of PTMs during pathogen-plant interactions are highlighted. We also summarize traditional tools and emerging proteomics approaches for PTM research. These discoveries open new avenues for investigating the fundamental infection mechanisms of plant pathogens and the discovery of novel strategies for plant disease control.

O-mannosyltransferase MaPmt2 contributes to stress tolerance, cell wall integrity and virulence in Metarhizium acridum

As a conserved post-translational modification, O-mannosyltransferase families play important roles in many cellular processes. Three subfamilies (MaPmt1, MaPmt2 and MaPmt4) are grouped in Metarhizium acridum according to sequence homology. The functions of MaPmt1 and MaPmt4 have been characterized in M. acridum previously. In this study, the functions of another member belonging to the Pmt2 subfamily, MaPmt2, were identified through RNAi strategy. The three MaPmt2 knockdown mutants showed dramatically decreased expression of MaPmt2. Phenotypic analyses showed that the mutants exhibited decreased tolerances to wet-heat, UV-B irradiation and cell wall perturbing chemicals. Further studies revealed that the mutants presented thinner cell walls observed by transmission electron microscope combined with changed cell wall components. Besides, knockdown of MaPmt2 decelerated conidial germination and decreased conidial yield. Compared with the wild-type strain, the MaPmt2 knockdown mutants caused impaired virulence only by topical inoculation. Results illustrated that the decreased virulence by inoculation could result from the delayed conidial germination on locust wings, reduced appressorium formation, as well as reduced turgor pressure in MaPmt2 knockdown mutants.

Protein glycosylation during infection by plant pathogenic fungi

Glycosylation is a conserved set of post-translational modifications that exists in all eukaryotic cells. During the last decade, the role of glycosylation in plant pathogenic fungi has received significant attention and considerable progress has been made especially in Ustilago maydis and Magnaporthe oryzae. Here, we review recent advances in our understanding of the role of N-glycosylation, O-glycosylation and glycosylphosphatidylinositol (GPI) anchors during plant infection by pathogenic fungi. We highlight the roles of these processes in regulatory mechanisms associated with appressorium formation, host penetration, biotrophic growth and immune evasion. We argue that improved knowledge of glycosylation pathways and the impact of these modifications on fungal pathogenesis is overdue and could provide novel strategies for disease control.

Structural, Evolutionary, and Functional Analysis of the Protein O-Mannosyltransferase Family in Pathogenic Fungi

Protein O-mannosyltransferases (Pmts) comprise a group of proteins that add mannoses to substrate proteins at the endoplasmic reticulum. This post-translational modification is important for the faithful transfer of nascent glycoproteins throughout the secretory pathway. Most fungi genomes encode three O-mannosyltransferases, usually named Pmt1, Pmt2, and Pmt4. In pathogenic fungi, Pmts, especially Pmt4, are key factors for virulence. Although the importance of Pmts for fungal pathogenesis is well established in a wide range of pathogens, questions remain regarding certain features of Pmts. For example, why does the single deletion of each pmt gene have an asymmetrical impact on host colonization? Here, we analyse the origin of Pmts in fungi and review the most important phenotypes associated with Pmt mutants in pathogenic fungi. Hence, we highlight the enormous relevance of these glycotransferases for fungal pathogenic development.

The completed genome sequence of the pathogenic ascomycete fungus Penicillium digitatum

P. digitatum, the causative agent of green mold, is one of the most destructive pathogens in the citrus industry. To facilitate basal researches on this important plant pathogen, here we report a finished genome sequence for P. digitatum strain PDW03 using a combination of Illumina, PacBio, and Hi-C sequencing technologies. The assembly comprised 6 chromosomes from telomere to telomere and encodes approximately 9000 proteins. Genomic re-analyses identified 302 Carbohydrate-active enzymes, 420 secreted proteins, and 39 secondary metabolite (SM) gene clusters. Furthermore, we found 10 fragmentary SM clusters in the P. digitatum PDW03 genome. Pangenome analysis based on 5 P. digitatum genomes available showed that conserved orthogroups account for ~68% of the species pangenome. Taken together, this fully completed P. digitatum genome will provide an optimum resource for further researches to investigate the driving forces of fungal host switch and effectors functioning in plant-pathogen interaction.

A Novel Secreted Cysteine-Rich Anionic (Sca) Protein from the Citrus Postharvest Pathogen Penicillium digitatum Enhances Virulence and Modulates the Activity of the Antifungal Protein B (AfpB)

Antifungal proteins (AFPs) from ascomycete fungi could help the development of antimycotics. However, little is known about their biological role or functional interactions with other fungal biomolecules. We previously reported that AfpB from the postharvest pathogen Penicillium digitatum cannot be detected in the parental fungus yet is abundantly produced biotechnologically. While aiming to detect AfpB, we identified a conserved and novel small Secreted Cysteine-rich Anionic (Sca) protein, encoded by the gene PDIG_23520 from P. digitatum CECT 20796. The sca gene is expressed during culture and early during citrus fruit infection. Both null mutant (Δsca) and Sca overproducer (Sca^op) strains show no phenotypic differences from the wild type. Sca is not antimicrobial but potentiates P. digitatum growth when added in high amounts and enhances the in vitro antifungal activity of AfpB. The Sca^op strain shows increased incidence of infection in citrus fruit, similar to the addition of purified Sca to the wild-type inoculum. Sca compensates and overcomes the protective effect of AfpB and the antifungal protein PeAfpA from the apple pathogen Penicillium expansum in fruit inoculations. Our study shows that Sca is a novel protein that enhances the growth and virulence of its parental fungus and modulates the activity of AFPs.

The Antifungal Protein AfpB Induces Regulated Cell Death in Its Parental Fungus Penicillium digitatum

Disease-causing fungi pose a serious threat to human health and food safety and security. The limited number of licensed antifungals, together with the emergence of pathogenic fungi with multiple resistance to available antifungals, represents a serious challenge for medicine and agriculture. Therefore, there is an urgent need for new compounds with high fungal specificity and novel antifungal mechanisms. Antifungal proteins in general, and AfpB from Penicillium digitatum in particular, are promising molecules for the development of novel antifungals. This study on AfpB’s mode of action demonstrates its potent, specific fungicidal activity through the interaction with multiple targets, presumably reducing the risk of evolving fungal resistance, and through a regulated cell death process, uncovering this protein as an excellent candidate for a novel biofungicide. The in-depth knowledge on AfpB mechanistic function presented in this work is important to guide its possible future clinical and agricultural applications.

Filamentous fungi produce small cysteine-rich proteins with potent, specific antifungal activity, offering the potential to fight fungal infections that severely threaten human health and food safety and security. The genome of the citrus postharvest fungal pathogen Penicillium digitatum encodes one of these antifungal proteins, namely AfpB. Biotechnologically produced AfpB inhibited the growth of major pathogenic fungi at minimal concentrations, surprisingly including its parental fungus, and conferred protection to crop plants against fungal infections. This study reports an in-depth characterization of the AfpB mechanism of action, showing that it is a cell-penetrating protein that triggers a regulated cell death program in the target fungus. We prove the importance of AfpB interaction with the fungal cell wall to exert its killing activity, for which protein mannosylation is required. We also show that the potent activity of AfpB correlates with its rapid and efficient uptake by fungal cells through an energy-dependent process. Once internalized, AfpB induces a transcriptional reprogramming signaled by reactive oxygen species that ends in cell death. Our data show that AfpB activates a self-injury program, suggesting that this protein has a biological function in the parental fungus beyond defense against competitors, presumably more related to regulation of the fungal population. Our results demonstrate that this protein is a potent antifungal that acts through various targets to kill fungal cells through a regulated process, making AfpB a promising compound for the development of novel biofungicides with multiple fields of application in crop and postharvest protection, food preservation, and medical therapies.

IMPORTANCE Disease-causing fungi pose a serious threat to human health and food safety and security. The limited number of licensed antifungals, together with the emergence of pathogenic fungi with multiple resistance to available antifungals, represents a serious challenge for medicine and agriculture. Therefore, there is an urgent need for new compounds with high fungal specificity and novel antifungal mechanisms. Antifungal proteins in general, and AfpB from Penicillium digitatum in particular, are promising molecules for the development of novel antifungals. This study on AfpB’s mode of action demonstrates its potent, specific fungicidal activity through the interaction with multiple targets, presumably reducing the risk of evolving fungal resistance, and through a regulated cell death process, uncovering this protein as an excellent candidate for a novel biofungicide. The in-depth knowledge on AfpB mechanistic function presented in this work is important to guide its possible future clinical and agricultural applications.

Citrus Postharvest Green Mold: Recent Advances in Fungal Pathogenicity and Fruit Resistance

As the major postharvest disease of citrus fruit, postharvest green mold is caused by the necrotrophic fungus Penicillium digitatum (Pd), which leads to huge economic losses worldwide. Fungicides are still the main method currently used to control postharvest green mold in citrus fruit storage. Investigating molecular mechanisms of plant-pathogen interactions, including pathogenicity and plant resistance, is crucial for developing novel and safer strategies for effectively controlling plant diseases. Despite fruit-pathogen interactions remaining relatively unexplored compared with well-studied leaf-pathogen interactions, progress has occurred in the citrus fruit-Pd interaction in recent years, mainly due to their genome sequencing and establishment or optimization of their genetic transformation systems. Recent advances in Pd pathogenicity on citrus fruit and fruit resistance against Pd infection are summarized in this review.

N-glycosylation of the protein disulfide isomerase Pdi1 ensures full Ustilago maydis virulence

Fungal pathogenesis depends on accurate secretion and location of virulence factors which drive host colonization. Protein glycosylation is a common posttranslational modification of cell wall components and other secreted factors, typically required for correct protein localization, secretion and function. Thus, the absence of glycosylation is associated with animal and plant pathogen avirulence. While the relevance of protein glycosylation for pathogenesis has been well established, the main glycoproteins responsible for the loss of virulence observed in glycosylation-defective fungi have not been identified. Here, we devise a proteomics approach to identify such proteins and use it to demonstrate a role for the highly conserved protein disulfide isomerase Pdi1 in virulence. We show that efficient Pdi1 N-glycosylation, which promotes folding into the correct protein conformation, is required for full pathogenic development of the corn smut fungus Ustilago maydis. Remarkably, the observed virulence defects are reminiscent of those seen in glycosylation-defective cells suggesting that the N-glycosylation of Pdi1 is necessary for the full secretion of virulence factors. All these observations, together with the fact that Pdi1 protein and RNA expression levels rise upon virulence program induction, suggest that Pdi1 glycosylation is important for normal pathogenic development in U. maydis. Our results provide new insights into the role of glycosylation in fungal pathogenesis.

The Myosin Motor Domain-Containing Chitin Synthases Are Involved in Cell Wall Integrity and Sensitivity to Antifungal Proteins in Penicillium digitatum

Penicillium digitatum is the main postharvest pathogen of citrus fruit and is responsible for important economic losses in spite of the massive use of fungicides. The fungal cell wall (CW) and its specific component chitin are potential targets for the development of new antifungal molecules. Among these are the antifungal peptides and proteins that specifically interact with fungal CW. Chitin is synthesized by a complex family of chitin synthases (Chs), classified into up to eight classes within three divisions. Previously, we obtained and characterized a mutant of P. digitatum in the class VII gene (ΔchsVII), which contains a short myosin motor-like domain (MMD). In this report, we extend our previous studies to the characterization of mutants in chsII and in the gene coding for the other MMD-Chs (chsV), and study the role of chitin synthases in the sensitivity of P. digitatum to the self-antifungal protein AfpB, and to AfpA obtained from P. expansum. The ΔchsII mutant showed no significant phenotypic and virulence differences with the wild type strain, except in the production and morphology of the conidia. In contrast, mutants in chsV showed a more dramatic phenotype than the previous ΔchsVII, with reduced growth and conidial production, increased chitin content, changes in mycelial morphology and a decrease in virulence to citrus fruit. Mutants in chsVII were specifically more tolerant than the wild type to nikkomycin Z, an antifungal inhibitor of chitin biosynthesis. Treatment of P. digitatum with its own antifungal protein AfpB resulted in an overall reduction in the expression of the chitin synthase genes. The mutants corresponding to MMD chitin synthases exhibited differential sensitivity to the antifungal proteins AfpA and AfpB, ΔchsVII being more susceptible than its parental strain and ΔchsV being slightly more tolerant despite its reduced growth in liquid broth. Taking these results together, we conclude that the MMD-containing chitin synthases affect cell wall integrity and sensitivity to antifungal proteins in P. digitatum.

PdMFS1 Transporter Contributes to Penicilliun digitatum Fungicide Resistance and Fungal Virulence during Citrus Fruit Infection

A new Penicillium digitatum major facilitator superfamily (MFS) transporter (PdMFS1) was identified and functionally characterized in order to shed more light on the mechanisms underlying fungicide resistance. PdMFS1 can play an important role in the intensification of resistance to fungicides normally used in P. digitatum postharvest treatments. In the PdMFS1 disrupted mutants, a slight effect in response to chemical fungicides was observed, but fungicide sensitivity was highly affected in the overexpression mutants which became resistant to wide range of chemical fungicides. Moreover, P. digitatum knock-out mutants exhibited a lower rate of fungal virulence when infected oranges were stored at 20 °C. Disease symptoms were higher in the PdMFS1 overexpression mutants coming from the low-virulent P. digitatum parental strain. In addition, the gene expression analysis showed an induction of PdMFS1 transcription in all overexpression mutants regardless from which progenitor came from, and four-time intensification of the parental wild type strain during citrus infection reinforcing PdMFS1 role in fungal virulence. The P. digitatum MFS transporter PdMFS1 contributes not only to the acquisition of wide range of fungicide resistance but also in fungal virulence during citrus infection.

Rational Design and Biotechnological Production of Novel AfpB-PAF26 Chimeric Antifungal Proteins

Antimicrobial peptides (AMPs) have been proposed as candidates to develop new antimicrobial compounds for medicine, agriculture, and food preservation. PAF26 is a synthetic antifungal hexapeptide obtained from combinatorial approaches with potent fungicidal activity against filamentous fungi. Other interesting AMPs are the antifungal proteins (AFPs) of fungal origin, which are basic cysteine-rich and small proteins that can be biotechnologically produced in high amounts. A promising AFP is the AfpB identified in the phytopathogen Penicillium digitatum. In this work, we aimed to rationally design, biotechnologically produce and test AfpB::PAF26 chimeric proteins to obtain designed AFPs (dAfpBs) with improved properties. The dAfpB6 and dAfpB9 chimeras could be produced using P. digitatum as biofactory and a previously described Penicillium chrysogenum-based expression cassette, but only dAfpB9 could be purified and characterized. Protein dAfpB9 showed subtle and fungus-dependent differences of fungistatic activity against filamentous fungi compared to native AfpB. Significantly, dAfpB9 lost the fungicidal activity of PAF26 and AfpB, thus disconnecting this activity from the fungistatic activity and mapping fungicidal determinants to the exposed loop L3 of AfpB, wherein modifications are located. This study provides information on the design and development of novel chimeric AFPs.

A highly efficient Agrobacterium tumefaciens-mediated transformation system for the postharvest pathogen Penicillium digitatum using DsRed and GFP to visualize citrus host colonization

Penicillium digitatum is a major postharvest pathogen of citrus crops. This fungus broadly spreads worldwide and causes green mold disease, which results in severe losses for citrus production. Understanding of the citrus infection by P. digitatum may help develop effective strategies for controlling this pathogen. In this study, we have characterized a virulent strain of P. digitatum isolated in Vietnam and established a highly efficient Agrobacterium tumefaciens-mediated transformation (ATMT) system for this fungal strain with two newly constructed binary vectors. These binary vectors harbor dominant selectable markers for hygromycin or nourseothricin resistance, and expression cassettes for the red fluorescent protein (DsRed) or the green fluorescent protein (GFP), respectively. Using the established ATMT system, the transformation efficiency of the Vietnamese strain could reach a very high yield of 1240±165 transformants per 10⁶ spores. Interestingly, we found that GFP is much better than DsRed for in situ visualization of citrus fruit colonization by the fungus. Additionally, we showed that the transformation system can also be used to generate T-DNA insertion mutants for screening non-pathogenic or less virulent strains. Our work provides a new platform including a virulent tropical strain of P. digitatum, an optimized ATMT method and two newly constructed binary vectors for investigation of the postharvest pathogen. This platform will help develop strategies to dissect molecular mechanisms of host-pathogen interactions in more detail as well as to identify potential genes of pathogenicity by either insertional mutagenesis or gene disruption in this important pathogenic fungus.

Efficient production and characterization of the novel and highly active antifungal protein AfpB from Penicillium digitatum

Filamentous fungi encode distinct antifungal proteins (AFPs) that offer great potential to develop new antifungals. Fungi are considered immune to their own AFPs as occurs in Penicillium chrysogenum, the producer of the well-known PAF. The Penicillium digitatum genome encodes only one afp gene (afpB), and the corresponding protein (AfpB) belongs to the class B phylogenetic cluster. Previous attempts to detect AfpB were not successful. In this work, immunodetection confirmed the absence of AfpB accumulation in wild type and previous recombinant constitutive P. digitatum strains. Biotechnological production and secretion of AfpB were achieved in P. digitatum with the use of a P. chrysogenum-based expression cassette and in the yeast Pichia pastoris with the α-factor signal peptide. Both strategies allowed proper protein folding, efficient production and single-step purification of AfpB from culture supernatants. AfpB showed antifungal activity higher than the P. chrysogenum PAF against the majority of the fungi tested, especially against Penicillium species and including P. digitatum, which was highly sensitive to the self-AfpB. Spectroscopic data suggest that native folding is not required for activity. AfpB also showed notable ability to withstand protease and thermal degradation and no haemolytic activity, making AfpB a promising candidate for the control of pathogenic fungi.

Unravelling the contribution of the Penicillium expansum PeSte12 transcription factor to virulence during apple fruit infection

Blue mould disease caused by Penicillium expansum infection is one of the most important diseases of pome fruit accounting for important economic losses. In the present study, the PeSte12 transcription factor gene was identified, and deletant mutants were produced by gene replacement. Knockout mutants showed a significant decrease of virulence during apple fruit infection. Virulence was affected by the maturity stage of the fruit (immature, mature and over-mature), and disease severity was notably reduced when the apples were stored at 0 °C. The ΔPeSte12 mutants resulted defective in asexual reproduction, producing less conidia, but this characteristic did not correlate with differences in microscopic morphology. In addition, the ΔPeSte12 mutants produced higher quantity of hydrogen peroxide than the wild type strain. Gene expression analysis revealed that PeSte12 was induced over time during apple infection compared to axenic growth, particularly from 2 dpi, reinforcing its role in virulence. Analysis of transcriptional abundance of several genes in ΔPeSte12 mutants showed that in most of the evaluated genes, PeSte12 seemed to act as a negative regulator during axenic growth, as most of them exhibited an increasing expression pattern along the time period evaluated. The highest expression values corresponded to detoxification, ATPase activity, protein folding and basic metabolism. Gene expression analysis during apple infection showed that 3 out of 9 analysed genes were up regulated; thus, PeSte12 seemed to exert a positive control to particular type of aldolase. These results demonstrate the PeSte12 transcription factor could play an important role in P. expansum's virulence and asexual reproduction.

Effect of Trichoderma-enriched organic charcoal in the integrated wood protection strategy

The gradual elimination of chromium from wood preservative formulations results in higher Cu leaching and increased susceptibility to wood decay fungi. Finding a sustainable strategy in wood protection has become of great interest among researchers. The objective of these in vitro studies was to demonstrate the effect of T-720-enriched organic charcoal (biochar) against five wood decay basidiomycetes isolated from strongly damaged poles. For this purpose, the antagonistic potential of Trichoderma harzianum (strain T-720) was confirmed among other four Trichoderma spp. against five brown-rot basidiomycetes in dual culture tests. T-720 was genetically transformed and tagged with the green fluorescent protein (GFP) in order to study its antagonistic mechanism against wood decay basidiomycetes. It was also demonstrated that T-720 inhibits the oxalic acid production by basidiomycetes, a well-known mechanism used by brown-rot fungi to detoxify Cu from impregnated wood. Additionally, this study evaluated the effect of biochar, alone or in combination with T-720, on Cu leaching by different preservatives, pH stabilization and prevention of wood decay caused by five basidiomycetes. Addition of biochar resulted in a significant Cu binding released from impregnated wood specimens. T-720-enriched biochar showed a significant reduction of wood decay caused by four basidiomycetes. The addition of T-720-enriched biochar to the soil into which utility poles are placed may improve the efficiency of Cr-free wood preservatives.

A Penicillium chrysogenum-based expression system for the production of small, cysteine-rich antifungal proteins for structural and functional analyses

Background

Small, cysteine-rich and cationic antifungal proteins (APs) from filamentous ascomycetes, such as NFAP from Neosartorya fischeri and PAF from Penicillium chrysogenum, are promising candidates for novel drug development. A prerequisite for their application is a detailed knowledge about their structure–function relation and mode of action, which would allow protein modelling to enhance their toxicity and specificity. Technologies for structure analyses, such as electronic circular dichroism (ECD) or NMR spectroscopy, require highly purified samples and in case of NMR milligrams of uniformly ¹⁵N-/¹³C-isotope labelled protein. To meet these requirements, we developed a P. chrysogenum-based expression system that ensures sufficient amount and optimal purity of APs for structural and functional analyses.

Results

The APs PAF, PAF mutants and NFAP were expressed in a P. chrysogenum ∆paf mutant strain that served as perfect microbial expression factory. This strain lacks the paf-gene coding for the endogenous antifungal PAF and is resistant towards several APs from other ascomycetes. The expression of the recombinant proteins was under the regulation of the strong paf promoter, and the presence of a paf-specific pre-pro sequence warranted the secretion of processed proteins into the supernatant. The use of defined minimal medium allowed a single-step purification of the recombinant proteins. The expression system could be extended to express PAF in the related fungus Penicillium digitatum, which does not produce detectable amounts of APs, demonstrating the versatility of the approach. The molecular masses, folded structures and antifungal activity of the recombinant proteins were analysed by ESI–MS, ECD and NMR spectroscopy and growth inhibition assays.

Conclusion

This study demonstrates the implementation of a paf promoter driven expression cassettes for the production of cysteine-rich, cationic, APs in different Penicillium species. The system is a perfect tool for the generation of correctly folded proteins with high quality for structure–function analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0586-4) contains supplementary material, which is available to authorized users.

The Pmt2p-Mediated Protein O-Mannosylation Is Required for Morphogenesis, Adhesive Properties, Cell Wall Integrity and Full Virulence of Magnaporthe oryzae

Protein O-mannosylation is a type of O-glycosylation that is characterized by the addition of mannose residues to target proteins, and is initially catalyzed by evolutionarily conserved protein O-mannosyltransferases (PMTs). In this study, three members of PMT were identified in Magnaporthe oryzae, and the pathogenic roles of MoPmt2, a member of PMT2 subfamily, were analyzed. We found that MoPmt2 is a homolog of Saccharomyces cerevisiae Pmt2 and could complement yeast Pmt2 function in resistance to CFW. Quantitative RT-PCR revealed that MoPmt2 is highly expressed during conidiation, and targeted disruption of MoPmt2 resulted in defects in conidiation and conidia morphology. The MoPmt2 mutants also showed a distinct reduction in fungal growth, which was associated with severe alterations in hyphal polarity. In addition, we found that the MoPmt2 mutants severely reduced virulence on both rice plants and barley leaves. The subsequent examination revealed that the fungal adhesion, conidial germination, CWI and invasive hyphae growth in host cells are responsible for defects on appressorium mediated penetration, and thus attenuated the pathogenicity of MoPmt2 mutants. Taken together, our results suggest that protein O-mannosyltransferase MoPmt2 plays essential roles in fungal growth and development, and is required for the full pathogenicity of M. oryzae.

Disruption of ku70 involved in non-homologous end-joining facilitates homologous recombination but increases temperature sensitivity in the phytopathogenic fungus Penicillium digitatum

The dominant mechanism to repair double-stranded DNA breaks in filamentous fungi is the non-homologous end joining (NHEJ) pathway, and not the homologous recombination (HR) pathway that operates in the mutation of genes by replacement of target DNA for selection cassettes. The key to improve HR frequency is the inactivation of the NHEJ pathway by eliminating components of its Ku70/80 heterodimeric complex. We have obtained ku70 mutants of Penicillium digitatum, the main citrus postharvest pathogen. The increased efficiency of HR in Δku70 strains was demonstrated by the generation of mutants in two different chitin synthase genes (PdchsII and PdchsV). P. digitatum Δku70 strains showed no differences from the parental strain in vegetative growth, asexual development or virulence to citrus fruit, when experiments were conducted at the optimal temperature of 24°C. However, growth of Δku70 strains at temperatures higher than 24°C demonstrated a detrimental effect in axenic growth and conidia production. These observations are in agreement with previous studies describing differences between ku70 mutants and their parental strains in some fungal species, and must be taken into account for future applications of the Δku approach to increase HR efficiency in fungi.